The ExoMars Trace Gas Orbiter (TGO) is a collaborative project between the European Space Agency (ESA) and the Russian Federal Space Agency (Roscosmos) to send an atmosphere research orbiter and the Schiaparelli demonstration lander to Mars in 2016 as part of the European-led ExoMars mission.[5][6] The mission will follow with the ExoMars rover in 2018 in which the 2016 launched TGO spacecraft will also operate as a communication link with Earth and the rover.

The Trace Gas Orbiter will deliver the ExoMars Schiaparelli EDM lander and then proceed with atmospheric mapping. A key goal of this mission is to gain a better understanding of methane and other atmospheric gases present in the Martian atmosphere that could be evidence for possible biological or geological activity. This research will also help select the landing site for the 2018 ExoMars rover which will search for biomolecules and biosignatures.

The challenge to discern the source of methane in the atmosphere of Mars prompted the independent planning by ESA and NASA of one orbiter each that would carry instruments in order to determine if its formation is of biological or geological origin,[14][15] as well as its decomposition products such as formaldehyde and methanol.

Attempted collaboration with NASA

NASA's Mars Science Orbiter (MSO) was originally envisioned in 2008 as an all NASA endeavor aiming for a late 2013 launch.[16][17] NASA and ESA officials agreed to pool resources and technical expertise and collaborate to launch only one orbiter.[18] The agreement, called Mars Joint Exploration Initiative, was signed on July 2009 and proposed to utilize an Atlas rocket launcher instead of a Soyuz rocket, which significantly altered the technical and financial setting of the European ExoMars mission. Since the ExoMars rover was originally planned to be launched along the TGO, a prospective agreement would require that the rover lose enough weight to fit aboard the Atlas launch vehicle with NASA's orbiter.[19] Instead of reducing the rover's mass, it was nearly doubled when the mission was combined with other projects to a multi-spacecraft programme divided over two Atlas V-launches:[18][20][21] the ExoMars Trace Gas Orbiter (TGO) was merged into the project, carrying a meteorological lander slated for launch in 2016. The European orbiter would carry several instruments originally meant for NASA's MSO, so NASA scaled down the objectives and focused on atmospheric trace gases detection instruments for their incorporation in ESA's ExoMars Trace Gas Orbiter.[6][17][22]

Under the FY2013 budget President Obama released on February 13, 2012, NASA terminated its participation in ExoMars due to budgetary cuts in order to pay for the cost overruns of the James Webb Space Telescope.[23][24] With NASA's funding for this project cancelled, most of ExoMars' plans had to be restructured.[25]

Collaboration with Russia

On March 15, 2012, the ESA's ruling council announced it will press ahead with its ExoMars program in partnership with the Russian space agency (Roscosmos), which plans to contribute two heavy-lift Proton launch vehicles and an additional entry, descent and landing system to the 2018 rover mission.[26][27][28][29][30]

Status

Under the collaboration proposal with Roscosmos, the ExoMars mission is split into two parts: the orbiter/lander mission in March 2016 that includes the TGO and a static lander build by ESA named Schiaparelli; this will be followed by the ExoMars rover mission in 2018 —also to be launched with a Russian Proton rocket.

As of January 2016, the 600 kg descent module Schiaparelli and orbiter have completed testing and are being integrated to a Proton rocket at the Baikonur cosmodrome in Kazakhstan.[31] The launch is scheduled for 14 March 2016.

Spacecraft yaw axis control to ensure the three faces containing the science payload remain cold

Mass

3,130 kg (6,900 lb)

Payload

Up to 135.6 kg (299 lb) of scientific instruments

Science

Scale model of ExoMars Trace Gas Orbiter displayed during Paris Air Show 2015

The TGO will separate from the ExoMars stationary lander and provide it with telecommunication relay for 8 sols after landing. Then the TGO will aerobrake for seven months into a more circular orbit for science observations and will provide communications relay for the ExoMars rover to be launched in 2018, and will continue serving as a relay satellite for future landed missions until 2022.[2]

The mission will characterise spatial, temporal variation, and localization of sources for a broad list of atmospheric trace gases:

Spatial and temporal variability: Latitude-longitude coverage multiple times in a Mars year to determine regional sources and seasonal variations (reported to be large, but still controversial with present understanding of Mars gas-phase photochemistry.)

NOMAD and ACS will provide the most extensive spectral coverage of Martian atmospheric processes so far.[34][38] Twice per orbit, at local sunrise and sunset, they will be able to observe the Sun as it shines through the atmosphere. Detection of atmospheric trace species at parts-per-billion (ppb) level will be possible.

CaSSIS is a high-resolution (4.5 m/pixel), colour stereo camera for building accurate digital elevation models of the Martian surface. It will also be an important tool for characterizing candidate landing site locations for future missions.

FREND is a neutron detector that can provide information on the presence of hydrogen, in the form of water or hydrated minerals, in the top metre layer of the Martian surface.[37]

Relay telecommunications

Due to the challenges of entry, descent, and landing, Mars landers are highly constrained in mass, volume, and power. For landed missions, this places severe constraints on antenna size and transmission power, which in turn greatly reduce direct-to-Earth communication capability in comparison to orbital spacecraft. As an example, the capability downlinks on Spirit and Opportunity have only 1/600th the capability of the Mars Reconnaissance Orbiter downlink. Relay communication addresses this problem by allowing Mars surface spacecraft to communicate using higher data rates over short-range links to nearby Mars orbiters, while the orbiter takes on the task of communicating over the long-distance link back to Earth. This relay strategy offers a variety of key benefits to Mars landers: increased data return volume, reduced energy requirements, reduced communications system mass, increased communications opportunities, robust critical event communications and in situ navigation aide.[39] NASA provided an Electra telecommunications relay and navigation instrument to assure communications between probes and rovers on the surface of Mars and controllers on Earth.[40] The TGO will provide the EDM lander and ExoMars rover with telecommunication relay and will continue serving as a relay satellite for future landed missions until 2022.[2]